The problem with which the invention is concerned is that, in order that the clutch release bearing can be engaged with the thrust member during fitting, the movable member of the hydraulic actuator which carries it must be in a withdrawn position with respect to the fixed member of the actuator, with which it is associated; in addition, in order that the clutch release bearing shall bear permanently against the thrust means during service through the coupling member, the movable member of the hydraulic actuator is, by contrast, permanently biassed towards its retracted position with respect to the fixed member of the actuator, by suitable preloading means. During the fitting operation it is therefore first necessary to withdraw the movable member of the hydraulic actuator and to hold it in its withdrawn position, and then to release it. In other words, it is necessary to provide retaining means which are adapted to hold the movable member of the actuator in the withdrawn position for so long as the retaining means are effective.
In French patent specification No. FR 2 651 846A, these retaining means comprise a retractable abutment element which can be so controlled as to provide a means for engaging the movable member of the hydraulic actuator, at the rear of the latter. This retractable abutment means calls for an additional specific operation, to remove it so that the movable member of the hydraulic actuator can be released. This adds complication to the fitting operations.
It should be noted that this also increases quite considerably the radial size of the assembly, and that the latter is not only relatively complex and costly to manufacture, but also provides a rigid mechanical abutment surface to the movable member of the hydraulic actuator, so that it is liable to cause damage to one or other of the two members of the hydraulic actuator if the axial thrust which is exerted in order to achieve the required snap-fitting is too great.